It is generally accepted that elastomers which are to be used in the manufacture of tires must satisfy a number of criteria. In particular, the cold creep of the elastomers must be as low as possible; the elastomers must be readily processable in subsequent blending processes; the elastomers must be flowable during the molding processes and the elastomers must be readily vulcanizable.
With the increasing demand for automobile safety and low fuel consumption, the specifications for tire tread elastomeric compounds have become more exacting. Tire treads are now required to retain their elastomeric characteristics over a wide temperature range, to have high abrasion resistance in order to provide a long life expectancy, to exhibit good anti-skid properties in both wet and dry conditions and to have low hysteresis characteristics at elevated temperatures, that is to have good rebound characteristics and resilience in order to provide low rolling resistance and reduced dynamic heat build-up.
These specifications for tire treads are partly contradictory in that in order to improve the rolling resistance and the dynamic heat build-up, the damping properties of the tire tread compounds must be lowered. These in turn affect the wet traction properties and thus allow but a small margin for alterations in tread compounding.
By subjecting elastomeric vulcanizates to torsional vibration tests, measurements of the mechanical loss factor tan delta can be made which give an indication of the hysteresis power loss in the vulcanizates as they flex. Measurements of tan delta at different temperatures when expressed in the form of a graph, afford a curve whose slope gives an indication as to the performance of the vulcanizate with respect to its viscoelastic properties.
In Kautschuk und Gummi, Kunstoffe 38, 178 (1985), K. H. Nordsiek has demonstrated that the aforesaid specifications for tire tread compounds can be met if the tan delta curve comprises a vibration damping range which is as wide as possible. Furthermore, the value of tan delta should be relatively low at temperatures above 70.degree. C. so that there is minimal dynamic heat build-up and also in the temperature range between 30.degree. C. and 70.degree. C. so that rolling resistance is at a minimum. In the temperature range between 0.degree. and 30.degree. C. the value of tan delta should be relatively high in order for there to be superior skid resistance and there should be a subsequent flat increase in the value of tan delta to a maximum value at the lowest possible temperature so that there is excellent abrasion resistance and the rubbery characteristics of the tire tread are maintained.
Homopolymers based on the conventionally employed monomeric raw materials such as butadiene, isoprene and styrene do not satisfactorily meet these requirements. For example, high cis-1,4-polybutadiene provides vulcanizates having good wear resistance and good resilience together with an acceptably low glass transition temperature but the vulcanizates are lacking in both wet and dry traction, except perhaps on ice. High vinyl polybutadienes, on the other hand, are deficient in wear resistance and resilience and the glass transition temperature is too high to permit their use in colder climates.
The blending of homopolymers and copolymers has also not proven to be completely satisfactory in that the resultant tire tread compounds do not exhibit the aforesaid spectrum of properties. For example, blends of emulsion and solution polymers such as blends of styrene-butadiene copolymers and high vinyl polybutadienes have good wet skid resistance and acceptably low rolling resistance but they are deficient in other properties such as abrasion resistance.
Accordingly there exists a need for elastomers which substantially satisfy the aforesaid desired properties.